It is known that alkylation of the side chain of alkylaromatic derivatives is a useful method for preparing styrene and similar derivatives such as p-vinyltoluene or p-t-butylstyrene. Such alkylation reactions of toluene, xylenes, or p-t-butyltoluene by methanol, formaldehyde, or methylal (dimethyl acetal of formaldehyde), using catalysts based on zeolites exchanged by various alkali metal cations and/or doped with boron or phosphorus have already been described; see for example U.S. Pat. Nos. 4,115,424, 4,140,726, 4,463,204, and 4,483,936; the articles by C. Lacroix et al., Journal de Chimie Physique, 1984, 81, pages 473-490; German Patent Application 3,316,929; and the article by J. M. Garces et al., "Catalysis by Acids and Bases," Imelik et al. Ed., Ser. Studies in Surface Sciences and Catalysis, 20, 67-74 (1985), Elsevier Science Publishers B. V., Amsterdam.
The best-studied reaction is the toluene alkylation reaction which generally yields mixtures of styrene, ethylbenzene, and decomposition products of the alkylation agent, and possibly various products from alkylation of the aromatic ring. To direct the selectivity of the reaction to formation of styrene and ethylbenzene, the use of X zeolites modified by exchange of sodium cations by lithium, potassium, rubidium, and/or cesium cations has been recommended. X zeolites exchanged by cesium have proved to be the most useful.
It is known that zeolites of the faujasite type have a highly organized crystal structure, whereby the basic element is a three dimensional lattice composed of a regular arrangement of SiO.sub.4 and AlO.sub.4.sup.- tetrahedra connected together by oxygen vertices. The arrangement of the tetrahedra in space forms polyhedra that delimit a network of channels, cages, and cavities in which the alkali metal cations are located at points where they balance the negative charges supplied by the AlO4.sup.- tetrahedra.
A lattice with a faujasite structure has 192 SiO.sub.4 or AlO.sub.4 - tetrahedra and its general formula may be written: EQU Mp.sup.+ (AlO.sub.2).sub.p (SiO.sub.2)q.multidot.rH.sub.2 O
wherein
M is an alkali metal, and p and q are whole numbers such that p+q=192,
r is zero or a whole number less than 270, depending on the degree of hydration.
In X zeolites, the ratio q/p (Si/Al) is less than 1.5 and generally between 1 and 1.5.
The X zeolites may be prepared for example by the method described in U.S. Pat. Nos. 2,882,244 and 3,251,897.
Gallosilicates with a faujasite structure that have a formula similar to the foregoing may be prepared in a similar manner by replacing Al by Ga (reference: R. M. Barrer, "Hydrothermal Chemistry of Zeolites," Acad. Press, Lond, 1982, pp. 282s). In the description below, the general reference to zeolites by convention encompasses both aluminosilicates and gallosilicates.
These zeolites are present in the form of powders.
In zeolites, the metal ions (generally sodium ions) can be exchanged with other alkali metal ions in certain proportions by being placed in contact with a salt or hydroxide solution of these alkali metal ions; see for example the prior art cited in the introduction, or the following references: Sidorenko et al., Dokl. Akad. Nauk SSSR, 173, 132 (1967); Yashima et al., J. Catal., 26, 303 (1972); Itoh et al., J. Catal., 72, 170 (1981); H. Sherry J. Phys. Chem., 70, 1158 (1966); H. Sherry, "Molecular Sieves Zeolites," I., Flanigen et al., Ed., A.C.S. Series, 101, 350 (1971).
It is also known that a number of authors have introduced metallic sodium into zeolites with sodium cations, for example by causing them to contact sodium vapors, or by mixing with sodium azide followed by heat treatment whereby the ozide decomposes; see in particular J. A. Rabo et al., Discuss. Faraday Soc., 41, 323 (1966); and L. R. M. Martens et al., "Preparation of Catalysts IV," B. Delmon et al., Ed., pages 531-541, Elsevier Science Publishers B.V., Amsterdam (1987). The latter authors used modified zeolites obtained, in butene isomerization reactions at room temperature.